Text cursor in QPlainTextEdit

I create a QPlainTextEdit and load a file. However, I do not get a position cursor (a vertical line), even not after a mouse click. Only after an edit I get the position cursor. What is needed to show the cursor right away or after a mouse click?

The window has the focus. It happens in a viewer with Readonly properties and also in an editor (not readonly). If I click with the mouse on a location no textcursor shows up. When I change the position using the keyboard (e.g. right arrow) the textcursor shows up correctly with the Qt::IBeamCursor.
Any more ideas?
Erik